In a backwards straight somersault you can decide whether you twist early or late. Twisting early means, that you induce the twisting movement before you rotatation hits 180° and twisting late means, that the twist starts after 180°. For simplicity twisting early in a backwards somersault means inducing the twist at approximately 135° and twisting late at 225°.

As a gymnast wants to twist counter-clockwise, he jumps of a trampoline with both arms outstretched over his head into a straight backwards somersault. He now can decide if he wants to twist early or late.

If he decides to twist early counter-clockwise he lowers down his left arm sideways against his body. As an example following picture:

If he decides to twist late counter-clockwise his right arm is lowered down sideways against his body.

Could somebody explain to me, why a gymnast twist counter-clockwise if he lowers his left arm early at 135° or his right arm late at 225°? What physical laws are involved and when do they apply?